Re: [PATCH v1] mm: memory-tiering: Fix PGPROMOTE_CANDIDATE counting

[Shiyang Ruan <ruansy.fnst@fujitsu.com>]

在 2025/8/29 17:08, Vlastimil Babka 写道:
> On 7/29/25 05:51, Ruan Shiyang wrote:
> 
> A process nit: your RFC v3 had:
> 
> From: Li Zhijian <lizhijian@fujitsu.com>
> 
> and this one doesn't.
> 
>> Goto-san reported confusing pgpromote statistics where the
>> pgpromote_success count significantly exceeded pgpromote_candidate.
>>
>> On a system with three nodes (nodes 0-1: DRAM 4GB, node 2: NVDIMM 4GB):
>>   # Enable demotion only
>>   echo 1 > /sys/kernel/mm/numa/demotion_enabled
>>   numactl -m 0-1 memhog -r200 3500M >/dev/null &
>>   pid=$!
>>   sleep 2
>>   numactl memhog -r100 2500M >/dev/null &
>>   sleep 10
>>   kill -9 $pid # terminate the 1st memhog
>>   # Enable promotion
>>   echo 2 > /proc/sys/kernel/numa_balancing
>>
>> After a few seconds, we observeed `pgpromote_candidate < pgpromote_success`
>> $ grep -e pgpromote /proc/vmstat
>> pgpromote_success 2579
>> pgpromote_candidate 0
>>
>> In this scenario, after terminating the first memhog, the conditions for
>> pgdat_free_space_enough() are quickly met, and triggers promotion.
>> However, these migrated pages are only counted for in PGPROMOTE_SUCCESS,
>> not in PGPROMOTE_CANDIDATE.
>>
>> To solve this confusing statistics, introduce this PGPROMOTE_CANDIDATE_NRL
>> to count the missed promotion pages.  And also, not counting these pages
>> into PGPROMOTE_CANDIDATE is to avoid changing the existing algorithm or
>> performance of the promotion rate limit.
>>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Juri Lelli <juri.lelli@redhat.com>
>> Cc: Vincent Guittot <vincent.guittot@linaro.org>
>> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Ben Segall <bsegall@google.com>
>> Cc: Mel Gorman <mgorman@suse.de>
>> Cc: Valentin Schneider <vschneid@redhat.com>
>> Reported-by: Yasunori Gotou (Fujitsu) <y-goto@fujitsu.com>
>> Suggested-by: Huang Ying <ying.huang@linux.alibaba.com>
>> Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
> 
> So the S-o-b from Li doesn't match anything now.
> You can either reinstate that "From: Li ..." or add a "Co-developed-by: Li
> ..." right above the "S-o-b: Li ..." - that's for you two to decide who is
> the main author.

Thanks for pointing out.  I wasn't aware of this.

I'd like to add a Co-developed-by tag:

Co-developed-by: Li Zhijian

Then, should I resend a new version with is tag added?  Or you will do that for me?


--
Best regards,
Ruan.

> 
> More details in Documentation/process/submitting-patches.rst
> 
>> Signed-off-by: Ruan Shiyang <ruansy.fnst@fujitsu.com>
>> ---
>> Changes since RFC v3:
>>    1. change the naming of new added stat to PGPROMOTE_CANDIDATE_NRL.
>>    2. improve the description of the two stats.
>> ---
>>   include/linux/mmzone.h | 16 +++++++++++++++-
>>   kernel/sched/fair.c    |  5 +++--
>>   mm/vmstat.c            |  1 +
>>   3 files changed, 19 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index 283913d42d7b..4345996a7d5a 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -230,7 +230,21 @@ enum node_stat_item {
>>   #endif
>>   #ifdef CONFIG_NUMA_BALANCING
>>   	PGPROMOTE_SUCCESS,	/* promote successfully */
>> -	PGPROMOTE_CANDIDATE,	/* candidate pages to promote */
>> +	/**
>> +	 * Candidate pages for promotion based on hint fault latency.  This
>> +	 * counter is used to control the promotion rate and adjust the hot
>> +	 * threshold.
>> +	 */
>> +	PGPROMOTE_CANDIDATE,
>> +	/**
>> +	 * Not rate-limited (NRL) candidate pages for those can be promoted
>> +	 * without considering hot threshold because of enough free pages in
>> +	 * fast-tier node.  These promotions bypass the regular hotness checks
>> +	 * and do NOT influence the promotion rate-limiter or
>> +	 * threshold-adjustment logic.
>> +	 * This is for statistics/monitoring purposes.
>> +	 */
>> +	PGPROMOTE_CANDIDATE_NRL,
>>   #endif
>>   	/* PGDEMOTE_*: pages demoted */
>>   	PGDEMOTE_KSWAPD,
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 7a14da5396fb..4022c9c1f346 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1940,11 +1940,13 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
>>   		struct pglist_data *pgdat;
>>   		unsigned long rate_limit;
>>   		unsigned int latency, th, def_th;
>> +		long nr = folio_nr_pages(folio);
>>   
>>   		pgdat = NODE_DATA(dst_nid);
>>   		if (pgdat_free_space_enough(pgdat)) {
>>   			/* workload changed, reset hot threshold */
>>   			pgdat->nbp_threshold = 0;
>> +			mod_node_page_state(pgdat, PGPROMOTE_CANDIDATE_NRL, nr);
>>   			return true;
>>   		}
>>   
>> @@ -1958,8 +1960,7 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
>>   		if (latency >= th)
>>   			return false;
>>   
>> -		return !numa_promotion_rate_limit(pgdat, rate_limit,
>> -						  folio_nr_pages(folio));
>> +		return !numa_promotion_rate_limit(pgdat, rate_limit, nr);
>>   	}
>>   
>>   	this_cpupid = cpu_pid_to_cpupid(dst_cpu, current->pid);
>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>> index a78d70ddeacd..bb0d2b330dd5 100644
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -1272,6 +1272,7 @@ const char * const vmstat_text[] = {
>>   #ifdef CONFIG_NUMA_BALANCING
>>   	"pgpromote_success",
>>   	"pgpromote_candidate",
>> +	"pgpromote_candidate_nrl",
>>   #endif
>>   	"pgdemote_kswapd",
>>   	"pgdemote_direct",
>